Amino acids play a central role both as building blocks of proteins and as intermediates in metabolism. The chemical properties of the amino acids of proteins determine the biological activity of the protein, which in turn catalyze the large majority of the reactions in living cells and control virtually all cellular process.
Tyrosine is a polar and very weakly acidic aromatic amino acid. Tyrosine plays an important catalytic role in the active site of some enzymes (e.g. the bacterial enzyme DNA gyrase) and can be chemically modified after it has been incorporated into a peptide chain. A kinase enzyme (e.g. Wee1 involved in control of the cell cycle in yeast) can chemically link a phosphate group via the hydroxyl oxygen in a process called phosphorylation. The process can be reversed by a phosphatase enzyme (e.g. Cdc25 which reverses the effect of Wee1). This type of modification of tyrosine is extremely important in the regulation of the activity of various proteins.
Tyrosine is the direct precursor to several important neurotransmitters, such as dopamine, norepinephrine, epinephrine, and L-dopa. Some of the functions regulated by these tyrosine-dependent neurotransmitters include mood, stress response, mental function, satiety and sex drive. Tyrosine is also an important component of hormones that are produced by the thyroid. These hormones are in turn vital for managing metabolism. Tyrosine is also required to form melanin, the dark pigment that provides protection from the harmful effects of ultraviolet light. Dairy products, meats, fish, wheat, oats, as well as many other foods contain tyrosine.
In humans, tyrosine is a non-essential amino acid synthesized from the essential amino acid phenylalanine. Most plants and microorganisms, on the other hand, can synthesize tyrosine. The first step in the synthesis of the aromatic amino acids, including tyrosine, is the condensation of erythrose-4-phosphate and phosphoenolpyruvate to 3-deoxy-D-arabinoheptulosonic acid-7 phosphate (DAHP). In Escherichia coli, this reaction is carried out by three isoenzymes, aroF, aroG, and aroH.
Chorismate is a central intermediate of the shikimate pathway, and a branch point for five different metabolic pathways in microorganisms, including aromatic amino acid synthesis. Tyrosine is synthesized from chorismate via three enzymatic reactions, mediated by chorismate mutase, prephenate dehydrogenase, and transaminase A. In E. coli, both the CM and PDH activities are located in a single, bifunctional protein known as the T-protein, which is a homodimer with a molecular weight of approximately 78,000 that is encoded by the tyrA gene. The CM and PDH domains are located on the N- and C-terminal of TyrA, respectively.
In the first reaction, chorismate undergoes a Claisen rearrangement to form prephenate, which is catalyzed by chorismate mutase (CM). In the second reaction, prephenate undergoes NAD+-mediated oxidative decarboxylation to p-hydroxyphenylpyruvate, which is catalyzed by prephenate dehydrogenase (PDH). Finally, p-hydroxyphenylpyruvate is transaminated by transaminase A to produce L-tyrosine. Tyrosine (Tyr) is an end product inhibitor of both CM and PDH, and induces aggregation of the T-protein diminishing potential yields of this important amino acid.